1. Field of the Invention
The present invention relates to an oscillator, a dielectric waveguide device and a transmitter incorporating the same. More particularly, the invention relates to an oscillator for use at a high-frequency such as a millimeter wave or a micro-wave, in which the temperature dependence of an oscillation frequency is small, and a dielectric waveguide device and a transmitter incorporating the same.
2. Description of the Related Art
A millimeter-wave oscillator is an example of a high-frequency oscillator. The millimeter-wave oscillator is used, for example, in a vehicle-collision-avoidance radar system. In a typical type of millimeter-wave oscillator, a solid-state oscillating element such as a Gunn diode is often used.
In this case, an oscillation signal outputted from a Gunn diode is guided to a dielectric resonator via a dielectric waveguide, which electromagnetically couples to the dielectric resonator. In this arrangement, the dielectric resonator acts as a primary radiator to radiate the oscillation signal to an object.
FIG. 8 shows an equivalent circuit diagram of an oscillator using a Gunn diode. The oscillator shown in FIG. 8 is a reflector-type oscillator.
The section surrounded by a dotted line is the equivalent circuit of the Gunn diode, which includes a negative resistor xe2x88x92R and a reactor L1 connected in parallel to a capacitor C1, and a capacitor C2 connected in parallel to the negative resistor.
In addition, the oscillator further includes an excitation waveguide with a characteristic impedance Z1 and an electric length 1, and a load.
FIG. 9 shows the temperature dependence of an oscillation frequency of the oscillator. As shown in the figure, the oscillation frequency is reduced as the temperature rises. In general, the oscillation frequency of a Gunn diode varies in the range of approximately 50 to xe2x88x92100 ppm/xc2x0 C. Such temperature dependence of the oscillation frequency is considered to stem from the temperature dependence of the negative resistor xe2x88x92R.
In radar systems, stabilization of the oscillation frequencies of oscillators is of critical concern.
For example, Japanese Unexamined Patent Publication No. 6-268445 provides an oscillator constituted of a Gunn diode and a metal strip resonator. In the arrangement of the oscillator, the output from the oscillator is transmitted to a nonradiative dielectric waveguide, near which is disposed a modulator in which a dielectric resonator is placed close to a variable capacitance diode mounted on a nonradiative dielectric member. The oscillation frequency of the output from the oscillator can be changed by controlling the modulator.
In addition, there is provided a method for controlling the oscillation frequency by changing a bias voltage applied to a Gunn diode according to the ambient temperature.
However, in the former method, the modulator including the dielectric resonator needs to be disposed in such a manner that the modulator is not in contact with the non-radiative dielectric waveguide. In the latter method, it is necessary to provide a temperature-compensating circuit for the bias voltage. Therefore, the conventional art has problems in terms of reduction in size of the device.
To address these problems, the present invention provides an oscillator, a dielectric waveguide device and a transmitter incorporating the same which includes a special temperature-compensating circuit in order to solve problems such as increase in size, reduction in productivity, and rise in cost.
According to an aspect of the present invention, there is provided an oscillator including an oscillating element whose oscillation frequency exhibits temperature dependence, a dielectric substrate, and a transmission line disposed thereon, which has an input section for receiving an output from the oscillating element and an output section. The dielectric constant of the dielectric substrate exhibits temperature dependence so as to reduce the temperature dependence of an oscillation frequency from the output section.
For example, as a typical oscillating element, a Gunn diode is used. The oscillation frequency of the Gunn diode is lowered as the ambient temperature rises. But by setting the dielectric constant characteristics of the dielectric substrate appropriately, the electric length of the transmission line formed on the substrate is shortened in response to the temperature rise. As a result, the oscillation frequency outputted from the transmission line is increased. In other words, the temperature dependence of the oscillation frequency of the Gunn diode can be compensated for by using changes in the dielectric constant of the dielectric substrate.
For example, when the oscillation frequency of an oscillating element is lowered as the temperature rises, it is preferable to use a dielectric substrate whose dielectric constant decreases in response to the temperature rise.
In the present invention, the temperature dependence of the oscillation frequency of an oscillator can be lessened by appropriately selecting a material used for the dielectric substrate. In this case, it is only necessary to control the physical properties of the dielectric substrate engaging with the output line of the oscillator. Since no addition of temperature-compensating peripheral circuit is required, a compact oscillator can be obtained.
In short, it is only necessary to select an appropriate dielectric substrate according to the oscillation-frequency characteristics of the oscillator, and a Gunn diode may be used as the oscillating element in the invention.
Moreover, when the output signal of the oscillator is guided to an appropriate transmission line and is further guided to a radiator such as a dielectric resonator, a transmitter having good temperature-compensating characteristics can be produced.
Other features and advantages of the present invention will be described below, with reference to the drawings.